Graphite has important electrochemical properties. Thus for natural graphites, which are available abundantly in nature, a reversible capacity of 372 mAh/g and a voltage plateau close to that of lithium have been established.
Graphite has been introduced into commercial lithium-ion batteries, as mentioned in the patent granted to Sanyo in the United States, under number U.S. Pat. No. 5,882,818. Its important characteristics, in addition to its low cost price, have made natural graphite a good candidate as anode in lithium-ion batteries. However, coating of uniform electrodes remains problematic due to the physical shape of these particles, which have the shape of flakes. For this reason, carrying out the coating effectively requires an additional calendering step when the electrodes for Li-ion batteries are manufactured (Energy Storage Systems for Electronics, by Testuya Osaka and Madhav Datta, (2000), page 125). The compactness density is low, which results in electrodes having greater thicknesses. The performances of the anode depend on the type of graphite and the physical shape of these particles. The efficiency of the first intercalation of the ion in graphite is dependent on the specific surface area and the edge surface fraction (K. Zaghib et al, J. Electrochemical Soc. 147 (6) 2110 to 2115, 2000). A low specific surface area is associated with a lower contribution of passivation film.
Natural graphite is found exclusively in the form of flakes, while artificial graphite can be found in the form of flakes, fibers or spheres. The flake shape has an elevated degree of preferential orientation which will induce anisotropy in the electrode. An orientation such as this reduces the intercalation kinetics of lithium across the edges. However, the only spherical carbon available on the market is Mesocarbon Microbeads MCMB processed at 2,800° C. by Osaka Gas (T. Kasuh et al., J. Power Source 68 (1997), 99). This carbon is an artificial graphite that requires costly processing at high temperature to be ordered, as well as complex synthesis that can increase its production cost. The maximum reversible capacity obtained with this artificial graphite is of the order of 280 mAh/g, which is low in comparison to the corresponding capacity of natural graphite, which is 372 mAh/g.
U.S. Pat. No. 6,139,990 of Kansai Netsukkagaku Kabushiki Kaisha granted on Oct. 31, 2000, describes graphite particles that are modified and rounded, having an almost spherical form, characterized in that their degree of circularity is greater than or equal to 0.86 and in that, using X-ray diffraction measurement, the peak of the intensity relationship between one face 002 (parallel to the graphite layers) and face 110 (perpendicular to the graphite layers), that serve as the random orientation index, must not be less than 0.0050. These particles have poor homogeneity as regards their granulometric distribution, which limits their use in electrochemical cells, especially with propylene carbonate as electrolyte. This represents a major disadvantage for low-temperature applications. A lack of safety of electrochemical cells incorporating such graphite particles is also noted.
Patent application EP-A-0,916,618 filed in the name of the OSAKA GAS Co. Ltd., on the other hand, describes a graphite material in which the formation of cavities has been optimized in order to increase the electrochemical capacity of the electrodes containing it. While these materials have become interesting with regard to their use in primary-type batteries, they are of little interest for other electrochemical applications, in particular because of the fragility of their structure and the resulting lack of stability for capacities greater than 400 mA/g.
The handling required for converting natural graphite into spherical graphite presents net advantages in comparison to the standard natural graphite present in the form of flakes, as well as in comparison to spherical artificial graphite (MCMB).
Thus a need existed for graphite-based particles in a stable form that can be compressed easily to the point of obtaining an elevated density, these particles presenting electrochemical capacities and anisotropies that are greater than or equal to those of the known forms of graphite particles. In particular, these particles make it possible to produce homogeneous and compact electrodes and also promote the use of PC.